(i) Technical Field
The present invention relates to semiconductor devices, and more particularly, GaN-based semiconductor devices.
(ii) Related Art
Semiconductor devices using a GaN-based semiconductor are used as power devices capable of operating at high frequencies and outputting high power. There is known an FET such as a high electron mobility transistor (HEMT) as a semiconductor device suitable particularly for amplifying operations in high-frequency bands, which may include microwaves, quasi-millimeter waves, or millimeter waves.
The HEMT using a GaN-based semiconductor (hereinafter referred to as GaN-based HEMT) is required to operate at high frequencies and output high power. This requirement may be achieved by increasing the mutual conductance and the breakdown voltage. For example, Japanese Patent Application Publication No. 2006-286740 (Document 1) discloses a GaN-based HEMT configured to have an AlGaN electron supply layer having a recess between a gate electrode and a drain electrode and another recess between the gate electrode and a source electrode.
The following document discloses a GaN-based HEMT having a GaN channel layer and an AlGaN electron supply layer configured to have a GaN cap layer on the AlGaN electron supply layer: T. Kikkawa, et al., “An Over 200-W Output Power GaN HEMT Push-Pull Amplifier with High Reliability”, 2004 IEEE MTT-S Digest, 2004, p. 1347-1350 (Document 2).